Correction of electromagnetic lenses



lMmh 27, 1945. L, MARTQN 2,372,443

CORRECTION OF" ELECTROMAGNETIC LENS Filed April 22, 1942 lhwcntor @L1/nm@ Patented Mar. 27", 1,945

g UNITED STATE CORRECTIN ETIG NSE f' j Ladislaus. Marton, Palo Alto.,Cali, assigner to Radio Corporation,of Americaa. corporation,

' of Delaware Application fAprl 22, 1942;Y S'eral`N-o;4f40,095

, `Claims.l (Cl. Z50-161) This invention relates generally to magnetic electron lenses, and particularly to the correction ofaberration-in the magnetic eld of such lenses.

Heretofo-re, it has been common to shape theferroniagnetic elements ofl such'lens systemsl` to.

focus substantially all of the electronspassing through the lens system. However, existing elec'- tromagnetic lenses possess aberrations which limit their resolving power. ThisV invention contemplates the use of diamagnetic elements suitably placed in the magnetic field to correct fordis'- torton or aberration inherent in the conventional magnetic electron lenses. Such diamag netic elements will tend to repel the portion of the magnetic eld passing therethrough and hence may be used to control the distribution of the field immediately adjacent thereto. Many substances are diamagnetic in varying degrees. Some of these materials are bismuth, aluminum, glass, pure copper, wood, paper and various plastics.

An object of this invention is to provide means for shaping the magnetic eld of an electron lens by the introduction of suitably shaped diamag-V netic elements or substances in the field.` Another object is to provide means for correcting aberration or distortion in a magnetic electron lens by the insertion of one or more diamagnetic elements in the magnetic field of the lens. Still another object is to provide a combination of ferromagnetic or paramag'netic and diamagnetic substances in the magnetic eld of an electron lens to correct for distortion common to the usual ferromagnetic element. An additional object is to provide means for coating magnetic lens elements with diamagnetic substances to alter slightly the shape and characteristics of their magnetic field.

The invention will be further described by reference to the drawing in which Figure 1 is conventionall type of magneticelectron lens.' A

an elevational view illustrating the principal magnetic iield between opposite magnetic poles; Figure 2 is an elevational view illustrating the distortion of the field by the introduction of diamagnetic elements; Figure 3 is an elevational view of a magnetic electron lens; Figure 4 is an elevational View of a magnetic electron lens embodying the invention; Figure 5 is an elevational View of another embodiment of the invention; and Figure 6 is an elevational view of a further modification of the invention. Similar reference characters are applied to similar elements in the drawing.

Referring to Fig. 1, the curved lines X represent the magnetic field between the poles N and S of amagnet. The-introduction ofl the diamag-netic elements-D', D" tend to constrict or intensify the magnet-ic field between thepole pieces N and S asl shown inFig. 2. It Will` be understood' that thefd'egree of intensication is intentionallyexagge'ratedfin the drawing better to illustrate the pheno meon.

Fig/."3 illustrates the aberration common to a typical electromagnetic lens E, consistingV offtwo magnets A and'v B,- the poles N'and S of which arejplacedl inopposition-andare arranged to form magnetic iields coaxial with the electron beam Y. The focusing of the electron beams, by the electron lens E, is shown by dash lines.y The foci of all electron paths do not coincide; typical foci, occurring at different points on the lens axis, being shown as Fi and F2.

Fig. 4 shows a magnetic electron lens including diamagnetic elements D and D', shaped and located to correct for vthe aberration inherent in the lens of Fig. 3. It will be seen that the foci of all electron beams tend substantially to coincide at F.

Fig. 5 shows a magnetic electron lens including diamagnetic elements D and D', similar in purpose, shape and location to elements D and D of Fig. Ll, but each composed of diamagnetic seotion F and paramagnetic or ferromagnetic section G.

Fig. 6 shows a magnetic electron lens in which the pole pieces S and N are coated with diamagnetic material H to correct for aberration in-y herent in the lens of Fig. 3.

. While the use described pertains principally to the correction of aberration in an electron lens, it will be obvious to one skilled in the art that by suitable design of the shapes and locations of one or more diamagnetic elements, a magnetic field can be altered in any desired manner. The principal limitation on the design of such devices is the limited degree of diamagnetism obtainable with known materials.

A modiiication of the invention is to provide a combination of diamagnetic and paramagnetic materials in the magnetic structure of the lens.

An additional modification is to coat at least part of the paramagnetic structure of the lens with a diamagnetic material to correct for aberration of the eld of the lens without such a coating.

passed therethrough, and means for correcting diamagnetic member in said magnetic field, said member being shaped concavedly toward and located in immediate proximity to the path of said beam through said applied iield for altering the conformation of said field in opposite sense to and to a degree substantially to correctfor aberrations in the electron focus of said magnetic field.

3. An electromagnetic lens, comprising in combination, means including ferro-magnetic lens elements forestablishing a magnetic field, means for applying said field to focus an electron beam passed therethrough, and means for correcting lens iield aberrations comprising diamagnetic elements disposed in said lens magnetic field adjacent to said rferro-magnetic lens elements, said diamagnetic elements being shaped concavedly toward and located in immediate proximity tothe path of said beam through said applied eld for altering the conformation of said field in opposite sense to and to a degree substantially to correct for said aberrations.

4.r An electromagnetic lens comprising, in combination, means including ferro-magnetic lens elements for establishing a magnetic field, means for applying said iield to focus an electron beam passed therethrough, and means for correcting lens field aberrations comprising a combination of diamagnetic and ferro-magnetic elements disposed in said lens magnetic eld, said combined elements being shaped concavedly toward and located in immediate proximity to the Path of said beam through said applied field for altering the conformation of said field in opposite sense to and to-a degree substantially to correct for said aberrations.' A

5. Anelectromagnetic lens comprising, in combination, means including ferro-magnetic lens elements for establishing a magnetic eld, means for applying said field to focus an electron beam passed therethrough, and means for correcting lens field aberrations comprising a coating of diamagnetic material on at least a portion of said ferro-magnetic lens elements in the magnetic iield of said lens, said coated portions of said lens elements being interposed toward and located in immediate proximity to the path of said beam through said applied field for altering the conformation of said field in opposite sense to and to a degree substantially to correct for said aberrations.

LADISLAUS MARTON. 

